What causes the dead zone of air on the leeward side of a dune?

The dead zone of air on the leeward side of a sand dune is caused by the steepening of the leeward slope as the dune grows. Eventually, the slope becomes so steep that the wind cannot be deflected sharply enough to follow the surface contour. Consequently, the wind separates from the dune, creating a sheltered area of stagnant or recirculating air known as a dead zone.1

This separation allows sand transported up the windward slope to fall into the dead zone. As this material accumulates and the slope reaches the angle of repose for dry sand—approximately 32° to 33°—it forms a slip face. Once this equilibrium is established, the dune maintains its shape and migrates forward as sand is eroded from the windward side and deposited on the leeward slip face.12